This invention relates to sample analysis and in particular to a new and improved spectrometer of the electro-opto-acoustic type, using capacitative rather than acoustic detection.
Recently, several methods have been developed to measure heat which is generated in gases, liquids and solids by the absorption of light energy. This energy is characteristic of resonantly absorbed light which, in turn, is characteristic of the type and number of atoms and molecules which are responsible for the absorption. Other processes which scatter light are not detected.
This method of measuring which provides a measure of the amount of atoms or molecules in the absorbing medium, has great practical advantages over simply measuring the amount of light removed from a collimated light beam because, when this amount is small, great precision of measurement and great stability of light source and detector are required.
Light scattered from the beam may be measured to provide a measure of the composition of the scattering material. However, this method is not satisfactory in many instances because the scattering will be due to many processes not specifically characteristic of the constituents of the sample (surface scattering, Rayleigh scattering, particulate scattering, etc.). Under many conditions, the resonantly absorbed light is not re-radiated, because the resonant process is the slowest to radiate of all those processes absorbing radiation. The amount of light scattered by this process is thus greatly reduced. Furthermore, resonantly scattered light is extremely difficult to separate from light scattered by other processes.
The heat deposited in a confined gas sample by resonant absorption causes its temperature to rise and this increases its pressure. If the gas sample is periodically illuminated, the oscillating pressure constitutes a sound wave which can be measured with a microphone. This effect has been used with both laser and non-laser light sources to measure the light energy resonantly absorbed by gases and solids in energy contact with a confined gas. Examples of and apparatus for this process are set forth in U.S. Pat. Nos. 3,820,901, 3,659,452, 3,893,771, and 3,911,276, and reference may be had to these patents for more details on the electro-opto-acoustic spectrometer.
It is an object of the present invention to provide for sample collection in a sample carrier at one location and analysis of the sample in a spectrometer at another location. It is a further object of the invention to provide method and apparatus for sample analysis wherein radiant energy is absorbed by a sample resulting in heating and expansion of the sample, with the expansion in a capacitor environment providing an ac signal which can be related to the composition of the sample.